Various heat-sensitive transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver halide photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180). Moreover, this system has advantages over silver halide photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like. Further with a recent trend of speeding-up in printing, the dye diffusion transfer recording system is getting to have such an advantage that a printing time necessary to discharge a head of one sheet is short.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
From the view point of improving coloring properties in this system, proposals to use various dyes are disclosed (see publications such as JP-A-7-232482 (“JP-A” means unexamined published Japanese patent application), JP-A-5-221161, JP-A-4-357088, and JP-A-62-55194). However, there is a problem that a rate of occurrence of image defect in the time when the obtained image is continuously processed is higher than that of silver salt photography that has a long history in the technical field of color print materials. Especially, with a mass scale-continuous processing, shriveling occurs in a ribbon, so that a problem of transfer failure arises. In addition, generation of blocking and sticking increases a rate of occurrence of printing failure, which results in worsening of profitability. Further, recently from the view point of improvement in profitability, a processing speed of the printer tends to be improved. In order to respond to such a trend, it is becoming important to have a passing suitability as the best mode. Under such the background, it has been studied to change the back surface of an ink sheet in order to improve the sticking (see JP-A-8-207461). Although there is a tendency that the sticking is further worsened by use of a high speed printer, a solution to improve the sticking has not yet found out. Although the blocking is also worsened by speeding-up, a solution by use of materials has not yet found out.
In this system of thermally transferring a dye, methods of controlling glass transition temperature (Tg) are proposed from the view point of improvement in coloring efficiency (see JP-B-7-29504 (“JP-B” means examined Japanese patent publication) and JP-A-2005-119280). In discussion about a printer, especially a passing property under the high speed conditions (transferring failure owing to occurrence of sticking, blocking and shriveling in a ribbon), it becomes an important factor to control Tg of a heat transfer layer of the ink sheet in an adequate relation ship to the image-receiving sheet in order to control a transport property between the ink sheet and the image-receiving sheet during transfer. Further in this system, although it has become ordinary to print yellow, magenta, cyan and the like in an area order, there is no knowledge relating to specific solution for Tg value balance among these colors. Further, with respect to the colored ink that is composed of a dye and a binder as primary components, although the colored ink is a system in which because an interaction between the dye and the binder is large, a simple additivity is unwarrantable, a specific solution relating to a specific control means for Tg of the whole heat transfer layers has not yet been found.